Create custom graph and access values from Python
After a bit of investigation it seems you can't use Graph Editor for "standalone" FCurves.
However, you can link them to any object and name them as you like. I used the default Cube in example below, but it might be as well an empty mesh.

Below script uses Cube as object that "holds" your curves within its animation_data.action. It creates them if they do not exist, and if they exists current one is picked.

import bpy

placeholder = bpy.context.scene.objects["Cube"]
if not placeholder.animation_data:
    placeholder.animation_data_create()
if not placeholder.animation_data.action:
    placeholder.animation_data.action = bpy.data.actions.get("curves") or bpy.data.actions.new("curves")
action = placeholder.animation_data.action

print(placeholder)
name = 'my_curve'

fcurves = action.fcurves
for f in fcurves:
    if f.data_path == name:
        break
else:
    f = action.fcurves.new( name )
    f.select = True
    f.keyframe_points.insert( 0, 0 )
    f.keyframe_points.insert( 100, 10 )


Then you have get to edit curve you can add points by duplicating existing ones using shift-d.



After shaping it you can use FCurve.evaluate() (API docs - bpy.types.FCurve). For example like this:

for i in range(100):
    print(i, f.evaluate(i))